1. Field of Invention
This invention relates generally to optical transmission systems or networks and more particularly to optical transmitters and optical receivers operated with a thermally floating wavelength grid of optical signal channels where the individual channel signals at the optical transmitter grid channels are not held to a fixed and predetermined wavelengths along a standardized wavelength grid, such as the recommended ITU wavelength grid via G.692.
2. Description of the Related Art
In a conventional dense wavelength division multiplexed (DWDM) communication system available today from telecommunication service provider equipment manufacturers, the generation of a considerable amount of heat is commonplace and is a major limitation to decreasing the size, power, and cost of such equipment. The use of monolithically integrated photonic devices, such as electro-absorption modulated lasers (EMLs), optical transmitter photonic integrated circuits (TxPICs) and optical receiver photonic integrated circuits (RxPICs), which incorporate multiple electro-optic functions into a single semiconductor chip, can significantly reduce the overall power requirements of an optical transmitter module and optical receiver module. See, for example, examples of such TxPICs and RxPICs in respective patent applications, Ser. No. 10/267,331, filed Oct. 8, 2002, published on May 22, 2003 as Pub. No. US 2003/0095737 A1, and Ser. No. 10/267,304, filed Oct. 8, 2002, published Feb. 19, 2004 as Pub. No. US 2004/0033004 A1, which applications are incorporated in their entirety herein by their reference. The large-scale integration of these types of photonic integrated circuits (PICs) provides a large increase in functionality with an associated significant reduction in overall power, weight, size and cost. Although this kind of integration has demonstrated reduced power consumption, the thermoelectric cooler (TEC) or a Peltier cooler are still required to cool these PIC chips and can consume up to four to ten times as much power as the chip or chips themselves that are being cooled to maintain their operation at a designated operating temperature so that the transmitter modulated sources maintain their emission wavelengths along a standardized wavelength grid. This large power consumption via the use of such coolers significantly diminishes the effects of improvements made in device power requirements of such large-scale integration devices. Furthermore, the additional power utilized by the TEC increases the required heat sink size, weight, and cost, often exponentially. Thus, there is a major reason, as well as technical challenge, to remove the requirement for a TEC in the operation of such PICs.
As employed throughout this disclosure, “modulated sources” means either a modulated semiconductor laser source or a cw operated semiconductor laser in combination with an external electro-optic modulator which may be independent or integrated together which are referred to as electro-optic (electro-absorption) modulator/lasers (EMLs).
A major challenge in realizing an uncooled DWDM optical transmitter is maintaining the operating wavelengths of the multiple on-chip laser diodes in spite of changing ambient temperatures. DWDM implies an accurate control of the transmitter wavelength, whereas changing environmental temperature in a TxPIC, for example, inherently works also to change the wavelength of the on-chip laser diode transmitters. In this disclosure, a new and dramatic DWDM system approach is disclosed where thermal control of PIC chips such as EMLs, TxPICs or RxPICs in an optical transponder or transceiver module is absence to allow these integrated semiconductor devices to operate uncooled on a continuous basis regardless of any change in the emission wavelengths of the on-chip laser diodes.